The subject matter disclosed herein relates to gas compressors. More particularly, the subject matter disclosed herein relates to reciprocating gas compressors having an inertia conservation feature.
Gas compressors may be broadly grouped as either dynamic or positive displacement gas compressors. Positive displacement type compressors increase gas pressure by reducing volume occupied by the gas. Positive displacement gas compressors operate by confining a fixed amount of gas in a compression chamber, mechanically reducing the volume occupied by the gas thereby compressing the gas, and passing the compressed the gas into a distribution network. The gas pressure increase corresponds to the volume reduction of the space occupied by the amount of gas. As used herein, the term gas includes substances in a gaseous state, substances in a liquid state, and mixtures comprised of substances having both a liquid and a gaseous state.
Positive displacement compressors mechanically reduce the volume occupied gas using either a reciprocating piston or rotating component. Reciprocating compressors successively compress volumes of gas by repetitively driving a compression piston into a compression chamber in a first direction, withdrawing the piston from the compression chamber in a second direction, and allowing a volume of gas to be compressed to occupy the chamber. Each time the piston moves into the compression chamber it sweeps a portion of the chamber, thereby reducing the volume of chamber occupied by the gas, and raising the pressure therein. The compressed gas then exits the chamber, the piston withdraws from the chamber, and a second charge of gas enters the chamber for a subsequent reciprocation of the piston.
Reciprocating compressors may be either single-acting or double-acting. Single-acting compressors, as described above, effect compression only when driving the piston in the first direction. Double-acting compressors include a compression chambers associated with both the front face and rear face of the compression piston, thereby effecting compression with piston movement in both the first and second direction.
Reciprocating compressors may also be either single-stage or multi-stage. In single stage compressors, the compressor compresses the volume of gas in a single mechanical operation—such as in the first piston movement described above. In multi-stage compressors, the compressor compresses the volume of gas in more than one mechanical operation—such as by compressing gas with the front face of the piston in the first movement described above, moving the compressed gas to the chamber associated with the rear face of the piston, and further compressing the gas with the rear face of the piston in the second movement described above. Still other multi-stage compressors include a plurality of compression pistons arranged to compress gas with a plurality of compression operations.
Reciprocating compressors that use pistons for compressing have several disadvantages. For example, the inertial forces associated with the reciprocating components are high in piston-equipped compressors. During successive reciprocations, the compressor drive accelerates the piston in one direction, stops it, and then accelerates it in the opposite direction. The more massive the piston assembly, the greater the force the drive need supply to accelerate and decelerate the assembly. And since the kinetic energy of the assembly is typically dissipated (and not conserved) at the end of the stroke, the compressor is inherently less efficient. Such energy loss can be particularly severe in compressors having comparatively short strokes, where the inertial loads associated with accelerating the piston assembly is the peak load imposed upon the drive assembly. As a result, the majority of the force produced by the compressor drive goes not into compressing gas, but rather into successively accelerating the piston assembly.
In high-pressure natural gas applications, compressors are typically rotary driven. Rotary drives, in turn, have a mechanical connection between the rotating drive and the piston that converts drive shaft rotation into piston linear translation—typically through use of a connecting rod. Connecting rods constrain compression operation such that the portion of the compression chamber swept by the piston is constant. Hence, for purposes of varying the volume of gas compressed without altering drive shaft speed, piston-equipped compressors include a turndown. The turndown alters compression chamber volume by the volume of the chamber within which the piston reciprocates—thereby altering the compression the gas within the chamber undergoes during each stroke. Turndowns present their own disadvantages, such as being time consuming to adjust and even requiring that the compressor be taken off line so that an operator may physically operate a crank to alter the compression chamber volume.
One alternative that provides an adjustable capacity compressor is a linear motor driven compressor. Such a compressor was proposed in the Advanced Reciprocating Compression Technology Final Report, SwRI Project No. 18.11052 prepared under DOE Award No. DE-FC26-04NT42269, Deffenbaugh et al. (the “ARCT Report”), dated December 2005. However, as concluded in the ARCT Report, while a linear motor could be used to drive a reciprocating compressor, current linear motor technology limits such compressors to smaller diameter cylinders, operating at slower speeds and with relatively long stroke lengths—therefore having lower capacity and being unsuitable for conventional natural gas distribution systems. These limitations are due in part to the limited amount of force achievable through existing linear motor technology and in part due to the above-described rod load inertial load requirements.
Accordingly, there is a need for a reciprocating compressor where drive force requirement is driven by the force required to compress the gas in the compression chamber rather than the inertial force required to accelerate the compression piston. There is also a need a reciprocating compressor having a large bore diameter with an associated drive force requirement within the capabilities of existing linear motor technology. Finally, there is a need for a reciprocating compressor having a short stroke length with an associated drive force requirement within the capabilities of existing linear motor technology.